UC San Diego Library – Scripps Digital Collection

The classification of the tunas throughout the world has remained unsatisfactory for many years due chiefly to the difficulties involved in comparing large specimens from many localities. On the eastern side of the Pacific are found several species which have not been clearly separated from those of the Western and Mid-Pacific. of these forms four play an important role in the fisheries of California, Mexico and Central America. The present study was undertaken in March 1940, to determine the geographical range of these species and the relationships between them and similar ones occurring in the Central, Western and Equatorial Pacific. This was the first essential step in a comprehensive investigation of the tuna populations supporting the California industry. In particular, it was necessary to explore the differences between the bluefin and the oriental tuna and to know whether or not the yellowfin tuna, the skipjack and the albacore are of the same species as those taken in Japanese and Hawaiian waters. If such proved to be the case additional studies would be required to determine if any intermingling occurred between the populations in the different localities. If on the other hand, the species proved to be distinct the Eastern Pacific population might be exploited without regard to the fisheries of Hawaii and Japan.

The only comprehensive work on the systematics of the Pacific tunas was published by Kishinouye. He found that separation of the various species required a careful study of the anatomy of these fish. To follow the approach laid down in his paper, a similar detailed anatomical treatment of the problem was required to compare those species supporting the California fishery with Kishinouye's descriptions. Although this work appears to be principally morphological, the great detail in which the anatomy of the Eastern Pacific tunas has been studied will form a firm foundation upon which investigations may be extended into lines more directly applicable to conservation.

Concerning the skipjack, this work has demonstrated that within the entire fishing area in the Eastern Pacific extending along the Central and North American coastline from the Equator to California and off-shore to include all the outlying islands, there is but a single species, and specimens from all these areas are furthermore individually indistinguishable from those obtained from Japan and the Hawaiian Islands and described as Katsuwonus pelamis.

In the case of the yellowfin tuna a similar conclusion was reached. A single species, Neothunnus macropterus, exists throughout this fishing area, and these fish are individually indistinguishable from the specimens obtained from the Hawaiian Islands, from Japan and from Peru. Here again the range of the species spans the Pacific and extends southward to Callao, on the Peruvian coast. Within this large area distinct populations may exist, but conclusions concerning this must await the analysis of all the data collected.

The albacore obtained from Japan and from the Hawaiian Islands likewise proved to be similar to the fish of the North American coastline. All must be considered of the same species, Thunnus germo, with a geographical distribution extending across the north temperate Pacific. The authors agree with Kishinouye that the albacore belongs in the genus Thunnus, and should not be separated therefrom.

The bluefin tuna from Southern and Lower California are essentially one species, and until adequate descriptions are available from all localities, must be assigned to the same species as Thunnus thynnus of the Atlantic. Similarly, lacking material from Japan with which to compare directly the local specimens, it was necessary to base this comparison upon these findings and Kishinouye's description of Thunnus orientalis. Although very similar to Thunnus thynnus, T. orientalis (the oriental bluefin) as described by Kishinouye, differs in several respects from the local bluefin, and pending a direct comparison it must be concluded, tentatively, that the two are different, and that the local species is limited in distribution in the Pacific to the Eastern, temperate waters.

Lack of time prevented similar studies on the bonito, Sarda velox and S. lineolata, and on the black skipjack, Euthynnus lineatus. It is hoped that these forms may be investigated at some future date. These species are not reported, however, from Mid- and Western Pacific waters and their distribution does not present as great complexities as the other four species mentioned above.

Reports from fishermen and an observation by Kishinouye concerning the occurrence of Parathunnus mebachi, in the Eastern Pacific have been confirmed. This fish is very similar to the yellowfin tuna, but differs from it in outline, in the length of the pectoral and the size of the head and eye. The body and head are deeper than in the yellowfin, the pectoral is longer and the eye is conspicuously larger. For this reason, and because the specific name means, in Japanese, "big-eye," we suggest as a common name for this fish, the "big-eyed tuna." The name is descriptive, and is, moreover, commonly applied by fishermen to this fish. The species was first described by Kishinouye from Japanese waters. Except that it is taken occasionally at the Galapagos Islands and sometimes at Guadalupe Island (Mexico) and in the vicinity, nothing is known of its habitat on this coast. Kishinouye states, p. 444, that it is: "probably widely distributed in the deeper layer of the subtropical region of the Pacific Ocean."

The existing literature does not permit a positive identification of the Pacific tunas. Thus, Jordan and Evermann separate the yellowfin into two species on the basis of the height of the "dorsal and anal lobes," and the length of the pectoral fin. Neothunnus macropterus, with higher dorsal and anal lobes and a longer pectoral, is supposedly abundant "along the coast of tropical Mexico from Cape San Lucas to the Galapagos * * *," whereas N. catalinae, "* * * the northern representative of Neothunnus macropterus, is like the latter in almost all respects except that the fins are less developed." However, there is no justification for this separation and it is impossible to classify any given specimen on this basis.

Kishinouye's monograph, "Contributions to the Comparative Study of the So-called Scombroid Fishes," contains the most complete descriptions extant of the Pacific tunas. When, however, specimens of yellowfin tuna from the American coast were compared with his description of N. macropterus, a number of important differences were found superimposed upon a general, fundamental similarity. To identify positively the yellowfin of this coast, and thus determine the geographical range of the species, it was necessary to secure specimens from Japan for a direct comparison. The results of this comparison indicated a number of discrepancies in Kishinouye's work. These will be discussed when necessary in the text.

Specimens of yellowfin tuna, skipjack and albacore were obtained from Japan and the Hawaiian Islands. Specimens of yellowfin tuna only were obtained from Peru. In regard to the origin of these fishes, the Hawaiian specimens were taken in the vicinity of Honolulu, and the Peruvian yellowfin were shipped from, and probably caught in the proximity of, Callao; but of the Japanese specimens it is known only that they were shipped by freight from Japan. This material was compared directly with a large collection of local specimens collected in the course of research trips aboard the "N. B. Scofield" to all parts of the extensive local fishing grounds. All specimens were frozen and retained in cold storage until needed. A comparison of the bluefin tuna from California and from Lower California was also made, and the big-eyed tuna is described for the first time from this coast.

Preparatory to this work a study was made of the literature and in particular of Kishinouye's monograph, and a list of all characters used in the classification of the tunas was compiled. Preliminary dissections of local specimens were then made and characters offering little promise and those not amenable to routine examination were rejected, and others added. The final list was then organized into a procedure standard for the dissection of each species, and a corresponding blank was prepared for every specimen. The method proved very satisfactory and is heartily recommended because it insures continuity and comparability of observations throughout. The comparisons were made as exhaustive as circumstances permitted, in the hope that this publication might serve as a basis for a thorough and permanent classification of the Pacific tunas.

Foreseeing the necessity of a more detailed population study of some species, a large number of both external and internal measurements and counts was added to the routine. These, however, will be reserved for a biometrical analysis in a future publication and not discussed herein. In addition routine sketches were made of every organ system examined and the final illustrations were prepared from these without embellishment. of the five species investigated, a representative specimen from each widely separated area (Japan, Hawaii, local, etc.), was photographed.